Photosensitive diode arrays or photodiodes are used in an assortment of applications including radiation detection, optical position encoding, and low light-level imaging, such as night photography, nuclear medical imaging, photon medical imaging, multi-slice computer tomography (CT) imaging, and ballistic photon detection. Typically, photodiode arrays may be formed as one- or two-dimensional arrays of aligned photodiodes, or, for optical shaft encoders, a circular or semicircular arrangement of diodes.
One disadvantage with conventional detection devices is the amount and extent of crosstalk that occurs between adjacent detector structures, primarily as a result of minority carrier current between diodes. The problem of crosstalk between diodes becomes even more acute as the size of the detector arrays, the size of individual detectors, the spatial resolution, and spacing of the diodes is reduced.
More specifically, crosstalk occurs when photogenerated carriers, that are a result of incident light upon an active area of an individual photodiode unit, are not completely collected via the electrical contacts of that particular photodiode unit. Thus, a number of photogenerated carriers, and more particularly, those that are generated in the portion external to the depletion region, diffuse away from their point of generation and get collected or captured by electrical contacts of neighboring photodiode units. Photogenerated charge carriers therefore “random walk” while diffusing laterally through a layer of semiconductor material until an active area, which may be located a significant distance away from the point of origin of the charge carrier, collects them. The end result is a form of signal noise resulting from crosstalk between the photodiodes, and is the principal cause of electrical crosstalk.
In certain applications, it is desirable to produce optical detectors having small lateral dimensions and spaced closely together. For example in certain medical applications, it would beneficial to increase the optical resolution of a detector array in order to permit for improved image scans, such as computer tomography scans. However, at conventional doping levels utilized for diode arrays of this type, the diffusion length of minority carriers generated by photon interaction in the semiconductor is in the range of at least many tens of microns, and such minority carriers have the potential to affect signals at diodes away from the region at which the minority carriers were generated. Therefore, the spatial resolution obtainable may be limited by diffusion of the carriers within the semiconductor itself, even if other components of the optical system are optimized and scattered light is reduced.
Various approaches have been used to minimize crosstalk including, but not limited to, providing inactive photodiodes to balance the leakage current and using conventional two-dimensional or three-dimensional structures, such as trenches, moats, or insulating structures between photodiodes or other active devices to provide isolation between the devices.
For example, U.S. Pat. No. 4,904,861, assigned to Agilent Technologies, Inc., discloses “an optical encoder comprising: a plurality of active photodiodes in an array on a semiconductor chip; a code member having alternating areas for alternately illuminating and not illuminating the active photodiodes in response to movement of the code member; means connected to the active photodiodes for measuring current from the active photodiodes; and sufficient inactive photodiode area on the semiconductor chip at each end of the array of active photodiodes to make the leakage current to each end active photodiode of the array substantially equal to the leakage current to an active photodiode remote from an end of the array”. Similarly, U.S. Pat. No. 4,998,013, also assigned to Agilent Technologies, Inc. discloses “means for shielding a photodiode from leakage current comprising: at least one active photodiode on a semiconductor chip; means for measuring current from the active photodiode; a shielding area having a photodiode junction substantially surrounding the active photodiode; and means for biasing the shielding area photodiode junction with either zero bias or reverse bias.”
U.S. Pat. No. 6,670,258, assigned to Digirad Corporation, discloses “a method of fabricating a low-leakage current photodiode array comprising: defining frontside structures for a photodiode on a front side of a substrate; forming a heavily-doped gettering layer on a back surface of the substrate; carrying out a gettering process on the substrate to transport undesired components from the substrate to said gettering layer, and to form another layer in addition to said gettering layer, which is a heavily-doped, conductive, crystalline layer within the substrate; after said gettering process, removing the entire gettering layer; and after said removing, thinning the heavily-doped, conductive, crystalline layer within the substrate to create a native optically transparent, conductive bias electrode layer”.
U.S. Pat. No. 6,569,700, assigned to United Microelectronics Corporation, discloses “a method of reducing leakage current of a photodiode on a semiconductor wafer, the surface of the semiconductor wafer comprising a p-type substrate, a photosensing area for forming a photosensor of the photodiode, and a shallow trench positioned in the substrate surrounding the photosensing area, the method comprising: forming a doped polysilicon layer containing p-type dopants in the shallow trench; using a thermal process to cause the p-type dopants in the doped polysilicon layer to diffuse into portions of the p-type substrate that surround a bottom of the shallow trench and walls of the shallow trench; removing the doped polysilicon layer; filling an insulator into the shallow trench to form a shallow trench isolation (STI) structure; performing a first ion implantation process to form a first n-type doped region in the photosensing area; and performing a second ion implantation process to form a second n-type doped region in the photosensing area”.
These prior art approaches, however, are typically not well suited to forming closely spaced miniaturized diode arrays, wherein the spacing between diodes should be in the range of a few microns. In addition, these prior art approaches require the use of complex processing and manufacturing steps that include the passivation of p-n junctions exposed by trenches between the active regions.
Many attempts have been made in the prior art towards isolating the active area of the photodiode array, including the use of 2D or 3D isolation structures and isolation films. For example, U.S. Pat. No. 6,826,080 by Park et al. discloses “a virtual ground nonvolatile semiconductor memory array integrated circuit structure comprising: a plurality of nonvolatile memory cells organized in a plurality of rows and columns, the memory cells being disposed in active areas of the integrated circuit; a plurality of row lines extending generally parallel to respective rows of the memory cells; a plurality of column lines extending generally parallel to respective columns of the memory cells; and an isolation structure disposed between each of the rows of memory cells and between adjacent columns of the memory cells at regular intervals throughout the memory array for electrically isolating the active areas from one another”.
Despite attempts in the prior art to improve the overall performance characteristics of photodiode arrays and their individual diode units, within detection systems, photodiode arrays capable of reducing crosstalk are still needed. Additionally, there is need for a semiconductor circuit and an economically feasible design and fabrication method so that it is capable of improving substantially reducing crosstalk.
In addition, there is a need for a front-side illuminated, back-side contact (FSL-BSC) photodiode arrays having superior characteristics, including low manufacturing cost via thermal budget processing; low crosstalk effects owing to active area isolation; and front to back intrachip electrical connections.